The present invention relates to a method of controlling heating resistors in a sublimation thermal transfer recording apparatus for performing recording having density of multiple gradations by using a thermal head.
In a known sublimation thermal transfer recording apparatus, a plurality of heating resistors are arranged in a line on a thermal head and are energized to be heated such that printing is performed on a recording medium. FIG. 1 shows an electric circuit of the known thermal head. The known thermal head includes heating resistors R.sub.i (i=0-1279 typically), a common resistor r.sub.1 and an FPC electrode resistor r.sub.2. From FIG. 1, a voltage V applied to the heating resistors R.sub.i is given by the following equation (a): EQU V=V.sub.H .times.R/(R+n.multidot.r) (a)
where V.sub.H denotes a voltage applied to the thermal head, R denotes a resistance value of each heating resistor, n denotes the number of the heating resistors R.sub.i driven simultaneously and r denotes a sum of a resistance value of the common resistor r.sub.1 and a resistance value of the FPC electrode resistor r.sub.2. It will be seen from the equation (a) that the voltage V is a function of the number n, the resistance value of the common resistor r.sub.1 and the resistance value of the FPC electrode resistor r.sub.2.
It is understood from the equation (a) as follows. Namely, in the case where the number n of the heating resistors R.sub.i driven simultaneously is increased, a so-called voltage drop phenomenon takes place in which the voltage V applied to the heating resistors is reduced unless the sum r of the resistance value of the common resistor r.sub.1 and the resistance value of the FPC electrode resistor r.sub.2 is minimized. As a result, the driven heating resistors Ri do not generate a desired quantity of heat, thereby resulting in the decrease of printed density.
In this known thermal head, while printing at a fixed density is being performed by using specific ones of the heating resistors Ri, printing at a density identical with that of the specific heating resistors is performed by the remaining heating resistors by gradually increasing the number of the remaining heating resistors subjected to heating. At this time, the solid lines in FIG. 2 show the relation before density correction between actual density of the specific heating resistors (ordinate) and the number of the remaining heating resistors subjected to heating (abscissa). In FIG. 2, assuming that the known sublimation thermal transfer recording apparatus enables recording of 128 graduations of print density, the indication 20", for example, represents the 20th gradation counted from the lightest gradation. In FIG. 2, the left ordinate represents optical density of the specific heating resistors, while the right ordinate represents gradation of the specific heating resistors. It will be seen from FIG. 2 that even if printing at a fixed density is performed by the specific heating resistors, printed density of the specific heating resistors linearly decreases from desired printed density as the number of the remaining heating resistors subjected to heating is increased gradually.
A method of correcting the decrease of printed density due to voltage drop of the heating resistors caused at the time of drive of the thermal head is disclosed in Chapter 4 of a book entitled "Sublimation dye transfer process" (1988). In order to implement the method, the sum r of the resistance value of the common resistor r.sub.1 and the resistance value of the FPC electrode resistor r.sub.2 is required to be reduced. To this end, a ceramic substrate of the thermal head is made larger in size, thereby resulting in rise of its production cost.
Therefore, so long as the sum r of the resistance value of the common resistor r.sub.1 and the resistance value of the FPC electrode resistor r.sub.2 is not reduced, decrease of printed density due to the above mentioned voltage drop should occur as the number of the heating resistors driven simultaneously is increased, so that the thermal head is incapable of outputting accurate printed density.